This invention relates to grinding apparatus and particularly to centerless grinding apparatus having feed mechanisms for introducing members to the grinder at a uniform rate.
The core of a nuclear reactor generally comprises an array or arrays of fuel assemblies which contain fuel elements. The fuel element is generally a cylindrical metallic sheath sealed at both ends containing nuclear fuel. The nuclear fuel which may be, for example, ceramic fuel pellets of a uranium compound, is stacked within the metallic sheath. During reactor operation, the nuclear fuel pellets fission releasing fission products such as fission gas while generating heat in a manner well known in the art.
There are many known methods for manufacturing the nuclear fuel pellets used in nuclear reactors. Most of these methods generally consist of cold pressing a powder which may be an oxide of fissionable material such as uranium dioxide to form dense compacts. These dense compacts are generally referred to as green pellets. The green pellets are then sintered in a non-oxidizing atmosphere to produce a sintered pellet which may have slight irregularities on its surface. The sintered pellet may then be ground to remove those irregularities thereby forming a right cylindrical pellet. This finished pellet is then stacked within the metallic sheath to form the fuel element that may be used in a nuclear reactor.
A commonly known method for producing the nuclear fuel pellets is described in U.S. Pat. No. 2,991,601 to J. Glatter et al, issued July 11, 1961. In this process, hydrogen reduction of uranium trioxide is employed to produce uranium dioxide powder. As received from commercial manufacturers, this uranium dioxide is not free flowing and is, therefore, not adaptable for use in automatic machinery for the production of the green pellets. In order to produce a free flowing powder, the uranium dioxide powder is mixed with a suitable binder such as aluminum stearate and water to form a wet granulate. The wet granulate is then forced through a screen and dried, after which it is dry-screened thereby separating the larger particles from the smaller particles. The water may be substantially removed in the later sintering process while the aluminum stearate will remain and act as a lubricant in the compacting process. Once the uranium dioxide powder has thus been converted into a free flowing granulate, the granulate is then compacted into green pellets in a cold pressing operation. The compacting process comprises flowing the granulate into a die and cold pressing the granulate in the die into substantially cylindrical green pellets. The green pellets may then be heat treated, sintered and ground to form the finished pellet for use in nuclear fuel elements.
During the sintering step in the manufacture of the nuclear fuel pellet, the pellet may shrink nonuniformly into a shape resembling an hour glass. A grinding process is then used to restore the cylindrical shape of the pellet. One known method of grinding fuel pellets comprises collecting the pellets in a vibratory bowl type pellet feeder, vibrating the pellets down a trough to the entrance of a centerless grinder where the pellets are ground to a proper shape. There are several problems associated with this concept. For example, the rate of vibration of the bowl type feeder changes with the changing pellet mass in the bowl thereby changing the feed rate. In addition, vibratory feeding results in sporadic pellet flow through the grinding apparatus resulting in various numbers of pellets in the grinder at any one time. The differing number of pellets present in the grinder causes uneven grinding pressure to be exerted on the pellets which results in nonuniformity of the pellets. It is, therefore, desirable to have a pellet feeder that separates the pellet stream and feeds pellets one at a time with each pellet having the same timing and velocity as it enters the grinding wheel. This would result in the same number of pellets being present in the grinding apparatus at all times thus allowing uniform grinding.